The invention concerns a magnetic resonance imaging method with which an image of an object under investigation, which is located in a homogeneous base magnetic field, is taken with which, for taking a single projection, a gradient magnetic field with a predetermined direction .phi. and strength G.phi. is applied, a high frequency excitation pulse is irradiated and, with a predetermined sampling rate i (i&gt;&gt;1), sequential measuring points S.sub.j of a nuclear resonance signal from the object under investigation which dephases under the influence of the gradient magnetic field G.phi. are subsequently measured which correspond to i points in k-space lying along a vector extending from the origin whose direction is determined by the direction .phi. of the gradient magnetic field, whereby, in k-space, the separation of each of the j-measuring points from the origin is given by the integral of the gradient magnetic field over the time interval between the excitation pulse and the taking of the jth-measuring point S.sub.j and, for further n-1 (n&gt;&gt;1) projections, the direction and/or strength of the gradient magnetic field is changed and the excitation, and measurement are repeated (n-1) times, whereby the image of the object under investigation is constructed from the n*i measuring points of all n projections according to a reconstruction algorithm.
A method of this type is, for example, known to those of skill in the art as the so-called projection reconstruction method (back projection) and now constitutes basic knowledge in the area of magnetic resonance (see for example the textbook "Nuclear Magnetic Resonance Imaging in Medicine and Biology" by P. G. Morris, Oxford Science Publications, Clarendon Press, Oxford, 1986, .sctn. 4.1).
A method is known in the art from the article "SPI-Single Point FID Imaging" by A. Nauerth and B. Gewiese, conference contribution to the 12.sup.th Annual Scientific meeting of SMR, 14th-20th Aug. 1993, New York, p. 1215, with which precisely one measuring point is taken after each high frequency excitation so that each point in k-space corresponds to one excitation ("Single Point Imaging"=SPI). The applicant's subsequently published German patent applications P 42 9 610.8, P 42 32 731.8 as well as P 43 34 038.5 likewise concern the so-called "SPI method" or variations thereof.
In the conventional imaging method the measurement signals are generally taken by measuring and digitizing a spin echo or a gradient echo signal subsequent to the high frequency excitation. Since one first allows the NMR signal to dephase following excitation and to rephase with the assistance of a 180.degree. pulse or through gradient inversion, one avoids the problem that, directly following excitation, the receiver is overloaded and a certain minimum amount of time t.sub.w must be waited before switching from transmission to reception. For this reason, with the original signal (FID), the initial measuring points of the NMR signal are not accessible to measurement. However, neglecting this signal portion would lead to enormous base line problems when Fourier transforming which renders good image reconstruction impossible. The echo signal solution is an elegant one and has significant advantages. However, this method increases the time interval between excitation and the taking of data which, in particular with investigational objects having short relaxation times T.sub.2, limits its applicability. Towards this end, the SPI method offers an alternative with which one can work with the shortest of intervals. However, this advantage is at the extreme expense of the total measuring time, since each single point in k-space must be measured individually. This cannot, in particular, be tolerated with three dimensional objects and/or biological or living samples.
A widely used alternative to the above mentioned back projection method which is likewise well known to those of skill in the art is the so-called 2dFT or 3dFT method with which the direction of a projection gradient is not changed rather, in addition to a read gradient, one or two changeable phase gradients are applied in the intervals between excitation and the taking of data. These methods, among others, are, for example, compared to each other in the textbook "Principles of Nuclear Magnetic Resonance Microscopy" by P. T. Callagahn, Clarendon Press Oxford, 1991, chapter 3. The Fourier methods have the advantage that k-space is evenly filled by the measuring points which accelerates the reconstruction of images from the measuring data (this is particularly important in the three-dimensional case) and, in general, leads to artifact free images. On the other hand the simple turning of a gradient with constant magnitude is often an experimentally more simpler method and can, in fact, be replaced with the rotation of a sample in a fixed gradient. In particular with the production of three-dimensional images it is important that Fourier method imaging, in particular with installations without array processors, is much faster than the back projection method. It can be carried out currently in acceptable times using installations which are only equipped with a PC. A three-dimensional back projection image (128*128*128 imaging points) requires, in this case, more than one hour. This cannot be tolerated in most cases.
It is therefore the object of the present invention to present a method which combines the advantages of the back projection method and those of the Fourier method with acceptable total measuring times including image reconstruction.